This invention relates generally to the fields of polymer chemistry, lithography, and semiconductor fabrication. More specifically, the invention relates to novel alicyclic polymers, particularly norbornene fluoroacrylate copolymers, which are useful in lithographic photoresist compositions, particularly chemical amplification photoresist compositions including ultraviolet, electron-beam, and x-ray photoresists.
There is a desire in the industry for higher circuit density in microelectronic devices made using lithographic techniques. One method of increasing the number of components per chip is to decrease the minimum feature size on the chip, which requires higher lithographic resolution. This has been accomplished over the past twenty years by reducing the wavelength of the imaging radiation from the visible (436 nm) down through the ultraviolet (365 nm) to the deep ultraviolet (DUV) at 248 nm. Development of commercial lithographic processes using ultra-deep ultraviolet radiation, particularly 193 nm, is now becoming of interest. See, for example, Allen et al. (1995), xe2x80x9cResolution and Etch Resist of a Family of 193 nm Positive Resists,xe2x80x9d J. Photopolym. Sci. and Tech. 8(4):623-636, and Abe et al. (1995), xe2x80x9cStudy of ArF Resist Material in Terms of Transparency and Dry Etch Resistance,xe2x80x9d J. Photopolym. Sci. and Tech. 8(4):637-642. The resists proposed for use with 193 nm imaging radiation do not appear suitable for use with 157 nm radiation due to their poor transparency at the 157 nm wavelength.
Certain attempts have been made to develop 157 nm resists, for example using heavily fluorinated materials such as polytetrafluoroethylene (e.g., Teflon AF(copyright); see Endert et al. (1999) Proc. SPIF-Int. Soc. Opt. Eng, 3618:413-417) or hydridosilsesquioxanes (see U.S. Pat. No. 6,087,064 to Lin et al.). These materials do not, however, have the requisite reactivity or solubility characteristics. The challenge in developing chemically amplified resists for 157 nm lithography is in achieving suitable transparency in polymers that have acid-labile functionalities and developed with industry standard developers in either exposed or unexposed areas depending on whether the resist is positive or negative.
Polymers prepared from trifluoromethyl-substituted acrylates have been described. See, for example, Ito et al. (1981), xe2x80x9cMethyl Alpha-Trifluoromethylacrylate, an E-Beam and UV Resist,xe2x80x9d IBM Technical Disclosure Bulletin 24(4):991, Ito et al. (1982) Macromolecules 15:915-920, which describes preparation of poly(methyl xcex1-trifluoromethylacrylate) and poly(xcex1-trifluoromethylacrylonitrile) from their respective monomers, and Ito et al. (1987), xe2x80x9cAnionic Polymerization of xcex1-(Trifluoromethyl)Acrylate,xe2x80x9d in Recent Advances in Anionic Polymerization, T. E. Hogen-Esch and J. Smid, Eds. (Elsevier Science Publishing Co., Inc.), which describes an anionic polymerization method for preparing polymers of trifluoromethylacrylate. Willson et al., Polymer Engineering and Science 23(18):1000-1003, also discuss poly(methyl xcex1-trifluoromethylacrylate) and use thereof in a positive electron beam resist. However, none of these references discloses the utility of trifluoromethyl-substituted acrylate polymers in chemical amplification resists.
Alicyclic polymers have also attracted a great deal of attention for their potential utility in advanced microelectronics technologies. The interest stems from their low dielectric constants and low UV absorption. Polymers of alicyclic monomers such as norbornene are typically prepared by metal-mediated addition or ring-opening metathesis polymerization (ROMP), which suffers from the drawbacks of high cost and possible metal contamination. Alternatively, the electron-rich norbornene monomers can be radically copolymerized with electron-deficient maleic anhydride to produce alternating copolymers, which have been heavily evaluated as 193 nm (ArF excimer laser) resist polymers. The norbornene-maleic anhydride co- and terpolymers can be readily prepared and tend to offer high performance lithographic imaging. However, these polymers tend to exhibit relatively high absorption at 157 nm. Furthermore, the maleic anhydride unit is of low functionality and therefore limits functionalization of such copolymers to only the norbornene monomer unit. It has been difficult to identify electron-deficient monomers that undergo radical copolymerization with norbornene derivatives. Such monomers have now been identified, and this invention now provides novel polymers that are synthesized by radical copolymerization of norbornene derivatives with comonomers such as xcex1-trifluoromethylacrylic acid and derivatives thereof.
Accordingly, it is a primary object of the invention to address the above-described need in the art by providing novel norbornene fluoroacrylate copolymers suitable for use in lithographic photoresist compositions.
It is another object of the invention to provide a lithographic photoresist composition containing a norbomene fluoroacrylate copolymer.
It is still another object of the invention to provide such a composition wherein the norbornene fluoroacrylate copolymer is substantially transparent to deep ultraviolet radiation, i.e., radiation having a wavelength less than about 250 nm.
It is yet another object of the invention to provide such a composition wherein the norbornene fluoroacrylate copolymer is a copolymer of norbornene or substituted norbornene and a fluorinated methacrylate, a fluorinated methacrylic acid, or a fluorinated methacrylonitrile.
It is still another object of the invention to provide a method for generating a resist image on a substrate using a photoresist composition as described herein.
It is a further object of the invention to provide a method for forming a patterned structure on a substrate by transferring the aforementioned resist image to the underlying substrate material, e.g., by etching.
Additional objects, advantages and novel features of the invention will be set forth in part in the description which follows, and in part will become apparent to those skilled in the art upon examination of the following, or may be learned by practice of the invention.
In one aspect, then, the present invention relates to a norbornene fluoroacrylate copolymer prepared by copolymerization of a first monomer having the structure (I) 
and a second monomer having the structure (II) 
In formula (I), r is zero or 1 and R1, R2, R3 and R4 are independently selected from the group consisting of hydrogen, hydroxyl, lower alkyl, fluorinated lower alkyl, xe2x80x94Oxe2x80x94(CO)-(lower alkyl), xe2x80x94CN, xe2x80x83xe2x80x94(L)nxe2x80x94CR12R13xe2x80x94OR7, andxe2x80x83xe2x80x83(VIII)

wherein m is an integer in the range of 1 to 3 and n is zero or 1. R7 is hydrogen, lower alkyl, fluorinated lower alkyl, or lower alkyl substituted with a tri(lower alkyl)silyl group, or is selected so as to provide an acid-cleavable substituent. R8 and R9 are lower alkyl or are linked to form a five- or six-membered heterocyclic ring that may or may not contain an additional heteroatom, R10 and R11 are lower alkyl or are linked to form a five-or six-membered heterocyclic ring that may or may not contain an additional heteroatom and/or a carbonyl moiety, n is zero or 1, and L is a linking group such as an alkylene (typically lower alkylene) chain or a phenylene ring. R14 is lower alkyl, fluorinated lower alkyl, or lower alkyl substituted with a tri(lower alkyl)silyl group, or is selected so as to provide an acid-cleavable substituent. Preferably, one of R1 through R4 has the structure of formula (III), and R7 is selected so as to provide an acid-cleavable functionality, i.e., a molecular moiety that is cleavable with acid, particularly photogenerated acid.
In formula (II), R5 is fluoro, methyl or trifluoromethyl, and R6 is xe2x80x94COOH, xe2x80x94CN, an amide, an acid-inert ester, or a functionality such as an acid-cleavable ester (e.g., (III), (IV) or (V)), an acetal or ketal (e.g., (VII), or a carbonate (e.g., (IX)). The polymer may serve as either the base-soluble component of an unexposed resist or as an acid-labile material (e.g., by virtue of containing acid-cleavable pendant groups such as acid-cleavable esters) that releases acid following irradiation by virtue of the photoacid generator in the resist composition.
In another aspect, the invention relates to a lithographic photoresist composition comprising a norbornene fluoroacrylate copolymer as described above, containing pendant acid-cleavable moieties, and a photosensitive acid generator (also referred to herein as a xe2x80x9cphotoacid generator,xe2x80x9d a xe2x80x9cPAG,xe2x80x9d or a xe2x80x9cradiation-sensitive acid generatorxe2x80x9d).
The present invention also relates to the use of the resist composition in a lithography method. The process involves the steps of (a) coating a substrate (e.g., a ceramic, metal or semiconductor substrate) with a film comprising a radiation-sensitive acid generator and a norbornene fluoroacrylate copolymer as provided herein; (b) exposing the film selectively to a predetermined pattern of radiation to form a latent image therein, and (c) developing the image using a suitable developer composition. The radiation may be ultraviolet, electron beam or x-ray. Ultraviolet radiation is preferred, particularly deep ultraviolet radiation having a wavelength of less than about 250 nm, e.g., 157 nm, 193 nm, or 248 nm. The pattern from the resist structure may then be transferred to the underlying substrate. Typically, the transfer is achieved by reactive ion etching or some other etching technique. Thus, the compositions of the invention and resulting resist structures can be used to create patterned material layer structures such as metal wiring lines, holes for contacts or vias, insulation sections (e.g., damascene trenches or shallow trench isolation), trenches for capacitor structures, etc. as might be used in the design of integrated circuit devices.